§ 202 ELECTRICAL ENGINEERING PRACTICE
is extremely small, this is true, and power can only be generated
where there happens to be an artificial fall (such as a lock or
weir) or a rapid capable of being converted into a fall. On the
other hand, the slope of rivers is generally greater, and an
artificial head can often be obtained by carrying the water for
some distance along a comparatively level artificial channel.
The horse-power available in any case is, theoretically, the pro-
duct of the weight of water in Ibs. per sec. (i.e. cu. ft. per sec.
x 62*3) multiplied by the vertical head in feet and divided by
550. The result must, however, be reduced in proportion to the
inefficiency of the turbine or wheel, and the loss of head in the
pipes, in order to find the actual B.H.P. available on the shaft.
The efficiency of modern wheels varies from about 65 up to over
90 °/o> but for rough calculations and including pipe losses 80 °/0
will not be far wrong. Taking this into account we have:—
Theoretical water H.P. = cu. ft. per sec. x head in ft. / 8*83.
Available B.H.P. = cu. ft. per sec. x head in ft. /11.
A cu. ft per sec. or 'cusec'—a useful irrigation term little
known outside India—will therefore give 0*09 B.H.P. on the
turbine shaft per ft. of fall; 0'9 B.H.P. per 10 ft. of fall;
9 B.H.P. per 100 ft.; and 90 B.H.P. per 1 000 ft. For rough
project estimates the overall efficiency of medium-sized electric
;enerators working at full load may be taken as 94 % (Chapter
iO); if driven otherwise than directly off the turbine shaft there
nil be an additional loss of about 5 °/0 in the drive. The
electrical power available at the generator terminals, assuming
this efficiency and converting from H.P. to kW, will then be
approximately:—
For direct drive: Kilowatts = cusecs x head in ft. / 15*5 (say 15).
For indirect drive: Kilowatts = cusecs x head in ft. /16'5 (say 16).
Thus, for example, we may have a canal fall of 10 ft. net with
1 000 cusecs flowing, or a mountain stream in which a fall of
1 000 ft. can be obtained with 10 cusecs flowing. In both cases:
The theoretical H.P. is 10 000 x 62*3/550 = 1 150; the turbine
B.ELP.is 10000/11 = 910; and the electrical power is 10000/15-5
or 16*5 = 645 or 605 kW, according as the drive is direct or in-
direct.
202. Constants and Approximations.—Table 30 and the
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